Alternating voltages are required between electrodes of matrix displays like LCDs, Plasma Display Panels (PDP), Plasma Addressed Liquid Crystal displays (PALC), and Electro-Luminescent panels (EL). Due to a capacitance present between the electrodes, and required steep slopes of the alternating voltage, relatively large charge or discharge currents are required to reverse the polarity of the voltage across the capacitance. To minimize the power dissipation during the polarity reversal, driver circuits which comprise an energy recovery circuit in which an external inductance forms a resonant circuit with the capacitance are known from EP-A-0548051 and EP-A-0704834. Both these prior arts disclose an energy recovery circuit for a PDP.
A PDP may be driven in a sub-field mode wherein, during a field or a frame of the video information to be displayed, a plurality of successive sub-fields or frames occurs. A sub-field comprises an addressing phase and a sustaining phase. During the addressing phase, the plasma rows are usually selected one by one and data in conformance with the video information to be displayed is written into pixels of the selected row. During the sustaining phase, a number of sustain pulses is generated dependent on the weight of the sub-field. Pixels pre-charged during the addressing phase to produce light during the sustaining phase will emit an amount of light during the sustaining phase which corresponds to the weight of the sub-field. The total amount of light produced by a pixel during the field or frame period of the video information depends, on the one hand, on weights of the sub-fields and, on the other hand, on the sub-fields during which the pixel was pre-charged to produce light.
In a PDP, the electrodes may be the scan electrodes and the common electrodes. Cooperating scan electrodes and common electrodes form pairs which are each associated with one of the plasma channels. During the sustaining phase, the pairs of electrodes are driven with anti-phase square-wave voltages generated by a full-bridge circuit. The full-bridge circuit comprises a first series arrangement of a first and a second controllable switch and a second series arrangement of a third and a fourth controllable switch. A junction of main current paths of the first and the second switch is coupled to a scan electrode. A junction of main current paths of the third and the fourth switch is coupled to a common electrode. The first series arrangement and the second series arrangement are arranged in parallel across terminals of a power supply source. The main current path of the first switch, is arranged between the scan electrode and a first one of the terminals, the main current path of the third switch, is arranged between the common electrode and said first terminal. During a first phase of a sustaining period, two of the switches are open while two of the other switches are closed, such that the power supply voltage supplied by the power supply source is available in a first polarity between the cooperating electrodes and thus across the capacitance. During a second phase of the sustaining period, the switches which were open during the first phase are now closed, and the switches which were closed are now open, such that the power supply voltage supplied by the power supply source is available in the reversed polarity between the cooperating electrodes.
A detailed description of this prior-art circuit and its operation is given in the description of FIG. 1 and FIG. 2.
Although the prior-art energy recovery circuit provides an efficient energy recovery, this circuit produces a considerable amount of Electro-Magnetic Interference (EMI).
It is, inter alia, an object of the invention to provide an efficient energy recovery circuit which produces less Electro-Magnetic Interference.
To this end, a first aspect of the invention provides an energy recovery matrix display driver circuit. Other aspects provide a matrix display apparatus comprising such an energy recovery matrix display driver circuit, and other advantageous embodiments.
At the end of a resonance period, when the current through the inductor changes polarity, this current has to follow a path that starts at one terminal of the inductor and ends at the other terminal of the inductor. In the prior art, this current has to flow via several diodes and one of the full-bridge switches (which is referred to as the second switch in the following description and in the claims). Thus, this current will flow through a loop with a large area and consequently generate a large electromagnetic field. As this second switch has to withstand a large voltage in a practical implementation, its impedance is quite high. Therefore, the voltage across the inductor will be quite high and thus an amount of energy stored in the inductor will be quite high. As the switch which connects the inductor and the capacitance to form a resonant circuit (this switch is referred to as the first switch in the following description and in the claims) has to be opened at or after the end of the resonance period to allow, at a start of the next resonance period, a change of the polarity of the voltage across the capacitive load in the opposite direction with respect to the first resonance period, the energy stored in the inductor will cause a high-frequency oscillation with a parasitic capacitance at the terminal of the inductor connected to the first switch.
The invention is based on the insight that this high-frequency oscillation is a major contributor to the EMI produced. In practice, the problem of the prior art is even more severe as the current in the loop through the second switch has to flow through two or three diodes, causing a voltage across the inductor which is the addition of two or three diode forward voltages and the voltage across the second switch.
In the circuit in accordance with the invention, an extra switch circuit is connected in parallel with the inductor to keep the above-mentioned current in a loop which is as small as possible. Furthermore, the switch circuit has to withstand a lower voltage than the second switch and will have a lower impedance in a practical implementation. But most importantly, the two or three diodes are not within the loop. Even if a unidirectional switch circuit is required, only one instead of two or three diodes is in the loop. Thus, in the circuit in accordance with the invention, the voltage across the inductor will be significantly lower than in the prior art. Consequently, the energy stored in the inductor is lower, and the EMI caused by the parasitic resonance will be significantly lower.
In an embodiment of the present invention, the switch circuit comprises a series arrangement of a diode and a controllable switch. This has the advantage over a controllable switch only that the timing of the on-time of the switch is less critical. It is no problem when the switch is on when the current through the inductor has such a polarity that the diode blocks.
In a further embodiment of the present invention, the energy recovery circuit has been made symmetrical to obtain an optimal efficiency in both resonance phases.
In yet another embodiment of the present invention, due to the presence of the switch circuit, it is possible to close the second switch at a later instant to prevent current flowing from the power supply voltage via the second switch at a later instant to prevent current flowing from the power supply voltage via the second switch to the capacitive load. In this way, less power is drawn from the power supply, and the efficiency even further improves.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.